Semiconductor devices are manufactured after performing many processes such as depositing a material layer on a wafer, patterning the deposited material layer, and removing unnecessary residuals on the wafer. To perform these processes repeatedly, a wafer is loaded on a wafer stage inside a chamber, the wafer is processed, and then unloaded.
In order to successively process a wafer, it is very important to chuck and fix the wafer in the chamber and to dechuck or remove the wafer so that the wafer will not be damaged after processing. As semiconductor devices become highly integrated, the design rule becomes smaller, and the process margin becomes narrower. As a result, there is a greater need to chuck and fix the wafer without damaging the wafer during dechucking.
Methods for fixing the wafer to the wafer stage in the process chamber when the wafer is loaded on the wafer stage include using hardware structures such as clamps, using a vacuum to suction the rear side of the wafer (a vacuum chuck), using gravity, and using a piezoelectric effect. Various methods are available for dechucking the fixed wafer on the wafer stage after processing the wafer. The dechucking method used is chosen in accordance with the method used for fixing the wafer.
The most widely used method for fixing a wafer is the piezoelectric effect. In this method, an electrostatic chuck is used to fix the wafer, and the electrostatic chuck and a lifting means are used to dechuck the fixed wafer.
FIG. 1 shows a sectional view of an electrostatic chuck and a wafer thereon in a process of chucking and dechucking the wafer according to the prior art. The electrostatic chuck 25 includes an upper insulating layer 5, an electrostatic electrode 10, a lower insulating layer 15, and a lower electrode 20. The lower electrode 20 controls the reaction speed of plasma when plasma is generated in the chamber (not shown). The electrostatic electrode 10 is connected to a DC generator (not shown), and positive charges or negative charges are distributed on the electrostatic electrode 10 by the DC generator. The electric charges on the electrostatic electrode 10 induce an electrostatic field such that the wafer 35 is chucked or dechucked. Wafer 35 and the electrostatic electrode 10 are insulated by the upper insulating layer 5.
In a method for chucking the wafer 35 according to the prior art, the wafer 35 is put on the electrostatic chuck 25, and an electrostatic field is induced by supplying power to the electrostatic electrode 10 under the upper insulating layer 5 on the upper surface of the electrostatic chuck 25. Positive charges accumulate on the electrostatic electrode 10 connected to the external DC generator (not shown), and negative charges accumulate on the lower surface of the wafer 35 by plasma generated on an upper portion of the wafer 35, thereby inducing an electrostatic field between the wafer 35 and the electrostatic electrode 10. When the upper surface of the electrostatic chuck 25 is completely in contact with the wafer 35, a clamping force is produced by the electrostatic field, and thus, the wafer is chucked.
In a process for dechucking the wafer 35, the voltage supplied to the electrostatic electrode 10 and the lower electrode 20 is turned off. As a result, the electric charges flow out and the clamping force is reduced. However, since a discharge time is necessary for the charges to flow when the clamping force is reduced, the wafer 35 becomes stuck to the electrostatic chuck 25.
When lift pins 30 are raised to dechuck wafer 35 that is stuck to the electrostatic chuck 25, the force applied to the wafer 35 can easily damage and/or break the wafer 35. It is often difficult to fully and efficiently dissipate the electric charge before the lift pins are raised to dechuck wafer 35.
For these reasons and other reasons that will become apparent upon reading the following detailed description, there is a need for an improved method and apparatus for safely dechucking wafers that avoids wafer breakage associated with conventional methods and apparatuses for dechucking wafers.